The present invention relates to superconductivity, more particularly to methods and devices using superconductivity for detecting or measuring magnetic fields or magnetic field gradients.
A “superconducting quantum interference device” (commonly referred to as a “SQUID”) is the most sensitive kind of magnetic field detector that is currently known. The material composition of a SQUID typically includes niobium and/or a lead alloy (e.g., lead-gold alloy or lead-indium alloy). The two main types of SQUID are “dc SQUID” (direct current SQUID) and “rf SQUID” (radio frequency SQUID). An rf SQUID is also referred to as an “ac SQUID” (alternating current SQUID). An rf SQUID has only one Josephson junction, whereas a dc SQUID has two or more Josephson junctions. Generally, dc SQUIDs are more sensitive than rf SQUIDS. The principle of operation of a SQUID relates to the physical phenomenon known as “flux quantization,” according to which a superconductor loop tends to favor states in which the flux inside the superconductor loop is a multiple of the flux quantum.
A typical dc SQUID includes a superconducting loop and two Josephson junctions that interrupt the superconducting loop at two respective locations. A “Josephson junction” consists of two superconducting regions and a non-superconducting region (e.g., a thin layer of insulating material) that is sandwiched between the two superconducting regions. A Josephson junction thus represents an interface between two superconducting materials that are separated by a non-superconducting (e.g., insulating) barrier. In a Josephson junction, electrons “tunnel” through the non-superconducting region according to the “Josephson effect.” When sufficient electrical current is conducted across a SQUID, a voltage is generated that is indicative of or related to the strength of a proximate magnetic field.
SQUIDs have been used in a variety of applications (e.g., scientific, medical, engineering, geological) demanding extreme sensitivity. A SQUID measures changes in a magnetic field with great sensitivity. However, SQUID sensitivity is subject to compromise by flux noise such as that which is associated with thermal noise and/or random vortex motion. The most sensitive SQUIDs available today are the dc SQUIDs having a low temperature superconductor (LTS) composition and operating at T=4.2K with liquid He as coolant. This dc SQUID describes a single superconductor loop discontinued by two Josephson junctions and ancillary structures such as a pickup coil. See, e.g., A. Barone and G. Paterno, Physics and Applications of the Josephson Effect, John Wiley and Sons, New York, 1982, incorporated herein by reference; J. Clarke, Poc. of IEEE 77, 1208 (1989), incorporated herein by reference; D. Koelle, R. Kleiner, F. Ludwig, E. Dankster and J. Clarke, Rev. Mod. Phys. 71, 631 (1999), incorporated herein by reference.